Digesters

Anaerobic Digestion

In anaerobic digestion, bacteria consume waste products, resulting in sanitization of the feed material, reduction of its mass, and release of methane gas that can be burned for heating or to generate electricity. Nutrient-rich byproducts of anaerobic digestion can also be used as fertilizer.

Is Anaerobic Digestion the Same as Composting?

Anaerobic digestion is similar to composting, except that it occurs in an oxygen-free (anaerobic) environment where bacteria that thrive without oxygen can multiply and digest the fuel material.

How Widespread is the Use of Anaerobic Digestion Today?

It is widely used as part of wastewater treatment.

Is Anaerobic Digestion a Feasible Method to Produce Power?

The technical expertise required to maintain industrial-scale anaerobic digesters, coupled with their high capital costs and efficiencies, have limited the level of its industrial application as a waste treatment and energy-production technology.

When Was Anaerobic Digestion Discovered?

In the seventeenth century, Robert Boyle and Stephen Hale noted that flammable gas was released by stirring the sediment of streams and lakes. In 1808, Sir Humphry Davy determined that methane was present in gases released from cattle manure.

The first anaerobic digester was built by a leper colony in Bombay, India in 1859. In 1895, in Exeter, England, a septic tank was used to generate fuel for gas lights.

Anaerobic digestion gained academic recognition in the 1930s, when research led to the discovery of anaerobic bacteria, the microorganisms that drive the process.

What Are the Best Applications for Anaerobic Digestion?

Anaerobic digestion is best-suited to treat wet organic material such as sewage. It can greatly reduce the amount of organic matter that might otherwise be either dumped at sea, landfilled, or burned in incinerators.

What Kinds of Fuels Can be Used for Anaerobic Digestion?

Almost any organic material can be processed by anaerobic digestion, including biodegradables such as paper, grass, food, sewage, and animal wastes. An exception is wood, which is largely unaffected by digestion. However, special anaerobic bacteria called lignin consumers have been used to break down woody materials and generate ethanol. Anaerobic digesters can also feed on specially grown energy crops such as silage to produce biogas (methane).

Is Small-Scale Energy Production from Anaerobic Digestion Practical?

In developing countries, simple home or farm-based anaerobic digestion systems offer the potential to produce low-cost energy for cooking and lighting. Since 1975, China and India have had large government-backed schemes to develop household biogas plants.

Can Anaerobic Digestion Help Reduce Environmental Pollution and Solid Wastes?

Pressure from environmental legislation in developed countries for solid waste disposal has increased the use of anaerobic digestion to reduce waste volume, while generating useful by-products.

Which is Better Anaerobic Digester Fuel, Sewage or Food Waste?

In Oakland, California, at the county wastewater treatment plant, food waste is digested together with municipal wastewater and other concentrated wastes. Anaerobic digestion of the food waste pulp provides more energy than municipal wastewater solids by volume: 730-1,300 kWh per dry ton of food waste, compared to 560-940 kWh per dry ton of municipal wastewater solids.

How Much Power Can an Anaerobic Digester Generate?

Biogas from sewage works is sometimes used to run a gas engine that produces electricity to help power and light the sewage works. Some of the waste heat from the engine is then used to heat the digester. However, the power potential from sewage works is limited – in the UK, for example, only about 80 MW total power is produced from sewage biogas, with potential to increase to 150 MW, an insignificant amount compared to the total UK power demand of about 35,000 MW.

The potential for biogas generation from non-sewage matter – e.g., energy crops, food waste, etc. – is much higher, estimated at 3,000 MW in the UK. Farm biogas plants that use animal waste and energy crops can reduce CO2 emissions, contribute to the electrical grid, and provide farmers with some revenue. Some countries now subsidize green energy production by offering incentives for feeding electricity from biogas into the power grid.

How Does Anaerobic Digestion Work?

In an anaerobic system, oxygen is prevented from entering the system by holding the waste fuel in sealed tanks. Digester bacteria (anaerobes) get oxygen directly from the organic material, or from inorganic oxides in the material. If the oxygen comes from the organic material, the end products are primarily alcohols, aldehydes, organic acids, and carbon dioxide. In the presence of inorganic oxides, the end products are methane, carbon dioxide, and traces of hydrogen sulfide.

Populations of anaerobic microorganisms typically take significant time to establish themselves and become fully effective. It is therefore common to introduce anaerobic microorganisms from materials with existing populations, “seeding” the digester by, for example, adding sewage sludge or cattle slurry.

What are the Stages of Anaerobic Digestion?

There are four key biological and chemical stages of anaerobic digestion:

1. Hydrolysis 2. Acidogenesis 3. Acetogenesis 4. Methanogenesis

Biomass is mostly composed of large organic polymers. For anaerobic digesters to access the energy in the material, the polymer chains must be broken down into their smaller components such as sugars, amino acids, and fatty acids.

Acetate and hydrogen from the first stages can be directly converted by the bacteria to methane. Other molecules, e.g., volatile fatty acids, must first be transformed into compounds that can be utilized by the methane-producing bacteria.

In the second stage, acidogenesis, the remaining components are further broken down into volatile fatty acids, ammonia, carbon dioxide, hydrogen sulfide, and other byproducts.

In the third stage, acetogenesis, simple molecules created during the acidogenesis phase are further digested by acetogens to produce acetic acid and some carbon dioxide and hydrogen.

In the final stage, methanogenesis, bacteria convert the end-products of the preceding stages into methane, carbon dioxide, and water, the major components of the biogas emitted from the system.

What Considerations Affect the Choice of Digester Raw Materials?

The most important issue for anaerobic digestion is the feedstock. Digesters can typically accept any biodegradable material, but if biogas is the goal, the level of putrescibility (rotting potential) is the key factor. The more putrescible the material, the higher the gas yield. Techniques are available to determine the composition of the feedstock.

Anaerobes can rapidly break down short-chain hydrocarbons such as sugars, but take longer to digest long-chain materials such as cellulose.

Anaerobic digesters were originally designed to consume sewage sludge and manures; however, these materials do not have the highest potential for anaerobic digestion, since the biodegradable material has had much of its energy content removed by the animal that produced it.

Therefore, many digesters use two or more types of feedstock. For example, a farm-based digester might use dairy manure as the primary feedstock, but land increase the gas production significantly by adding grass and corn. Other useful co-feedstocks include slaughterhouse wastes; fats, oils, and grease from restaurants; and organic household waste.

Also, the wetter the material, the more suitable it is to handling with standard pumps instead of energy-intensive concrete pumps, etc. With a high-solids anaerobic digester, bulking agents such as compost need to be added to increase the solid content of the input material.

Another key consideration is the carbon-nitrogen ratio of the input material. The optimal C:N ratio for a microbe is 20:1 to 30:1. Excess nitrogen can lead to ammonia inhibition of digestion.

Another key consideration is contamination of the feedstock material. If the feedstock has significant levels of physical contaminants, such as plastic, glass, or metals, pre-processing is required. If the contaminants aren’t removed, the digesters can be blocked and function inefficiently. The higher the level of pre-treatment required, the more processing machinery is required and the higher the capital costs of the plant.

After sorting or screening to remove physical contaminants, the material is often shredded, minced, and pulped to increase the surface area available to microbes in the digesters, thereby increasing the speed of digestion.

What Are the Types of Digesters?

Digesters can be configured with two levels of complexity: single-stage or multistage.

In a single-stage digester, all the biological reactions occur in a single sealed reactor or holding tank. A single-stage digester reduces construction costs; however, it gives less control of the chemical reactions.

Anaerobic digestion is a delicate process requiring careful analysis of bacteria, feedstocks, and process design. To give an example, acidogenic bacteria produce acids that reduce the pH of the tank. But methanogenic bacteria can only operate in a strict pH range. Thus, the biological reactions of the various bacteria species in a single-stage reactor can be in direct competition with each other.

Another one-stage reaction system is an anaerobic lagoon. These are pond-like earthen basins used for treatment and long-term storage of manures. Here the anaerobic reactions are contained within the natural anaerobic sludge in the pool.

In a two-stage or multi-stage digester, various digestion vessels are optimized for maximum control of the bacterial communities. For example, acidogenic bacteria more quickly grow and reproduce than methanogenic bacteria, but methanogenic bacteria require a stable pH and temperature to optimize their performance.

Typically, hydrolysis, acetogenesis, and acidogenesis occur in the first vessel. The organic material is then heated to the required temperature and pumped to a methanogenic reactor.

What Good and Bad Byproducts Do Anaerobic Digesters Produce?

Biogas may require treatment to “scrub” it before it can be used as fuel. For example, environmental agencies strictly limit the production of gases containing hydrogen sulfide. If hydrogen sulfide levels are high, gas scrubbing and cleaning equipment will be needed to process the gas. Alternatively, ferrous chloride (FeCl2) can be added to the digestion tanks to inhibit hydrogen sulfide production.

Volatile siloxanes can also contaminate the biogas. These compounds are often found in household waste and wastewater. In digestion facilities that accept such materials, low-molecular-weight siloxanes volatilize into biogas. When this gas is burned in a gas engine, turbine, or boiler, the siloxanes are converted to silicon dioxide (Si02) which deposits in the machine, increasing wear. Practical, cost-effective technologies for removing siloxanes and other biogas contaminants are available.

Digestate is the solid remains of the foodstock that the microbes can’t digest. It includes the mineralized remains of dead digestive bacteria. Digestate comes in three forms: fibrous, liquor, or sludge.

Digestate typically contains elements such as lignin that cannot be broken down by the anaerobic microorganisms. Also, the digestate may contain ammonia that is phytotoxic and will hamper the growth of plants if used as fertilizer. Thus, a maturation or composting stage may be necessary after digestion. Lignin and other materials can be degraded by aerobic microorganisms such as fungi to reduce the volume of the material. During maturation, the ammonia is broken down into nitrates, improving the value of the material as a fertilizer and soil improver.

Another byproduct is a stable organic material comprised mainly of lignin, cellulose, and mineral components in a matrix of dead bacterial cells (some plastic may also be present). This material resembles domestic compost and can be used as compost, or to make low-grade building products such as fiberboard.

Yet another byproduct is a nutrient-rich liquid that can be used as fertilizer, depending on the quality of the material being digested. The level of potentially toxic elements in this material must be assessed, and will depend on the quality of the original feedstock. Toxin levels are generally higher in industrial wastes than household refuse.

The final output from anaerobic digestion systems is water. The water comes from the moisture content of the original waste, and water produced by microbial reactions in the digester.

The wastewater from a digester will typically have elevated levels of biochemical and chemical oxygen demand, measures of the reactivity of the effluent and its ability to pollute. Some of this material will be “hard COD,” meaning that it cannot be converted to biogas. If the effluent were poured into waterways, it would pollute them by causing eutrophication. Thus further treatment of the wastewater is often required by passing air through it, or processing it in batch reactors or by a reverse osmosis unit.